(1) Field of the Invention
The present invention relates to a display device comprising TFTs (Thin Film Transistor) that use oxide semiconductors.
(2) Description of the Related Art
A liquid crystal display device or an organic EL display device uses TFTs for switching elements in the pixels or for the built in driving circuits. The TFT uses one of a-Si (amorphous Silicon), poly-Si (poly Silicon) or oxide semiconductor as an active layer.
The a-Si has low mobility; consequently, there are some problems to use the a-Si in the TFTs for the peripheral driving circuits. The poly-Si has high mobility, which is suitable for the TFTs for the peripheral driving circuits; however, the poly-Si has some problems for the switching TFTs in the pixels since it has bigger leak current. The oxide semiconductor has low leak current and the mobility is higher than the mobility of the a-Si; however, it has some problems of reliability in controlling defects in the semiconductor layer.
The patent document 1 (Japanese patent laid open 2012-15436) discloses the structure that the entire of the TFT, which comprises the oxide semiconductor and gate electrode, is covered by the inorganic insulating film of e.g. aluminum oxide, titanium oxide or indium oxide.
The patent document 2 (Japanese patent laid open 2015-92638) discloses the structure to suppress the gate leak caused by the tunnel effect when the gate insulating film becomes thin. The patent document 2 discloses to use the material of high dielectric constant as e.g. hafnium oxide, tantalum oxide laminated with silicon oxide, silicon nitride or aluminum oxide, etc. for the gate insulating film.
The patent document 3 (WO 2010/041686) discloses to sandwich the channel of the oxide semiconductor by the inorganic insulating film to stabilize the characteristics of the TFT. The patent document 3 discloses to use e.g. aluminum oxide, titanium oxide or indium oxide for the inorganic insulating film.